WO2003023421A1

WO2003023421A1 - Capacitance measuring circuit, capacitance measuring instrument, and microphone device

Info

Publication number: WO2003023421A1
Application number: PCT/JP2002/009138
Authority: WO
Inventors: Masami Yakabe; Naoki Ikeuchi
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2001-09-06
Filing date: 2002-09-06
Publication date: 2003-03-20
Also published as: CN1271418C; KR20040041603A; TWI221195B; NO20032012L; CN1551990A; KR100715062B1; US20050017737A1; EP1426773A4; EP1426773A1; NO20032012D0; US7019540B2

Abstract

A capacitance measuring circuit (10) comprising an AC voltage generator (11), an operational amplifier (14) having a noninverted input terminal connected to a predetermined potential (in this example, to the ground), an impedance converter (16), a resistor (R1) (12) connected between the AC voltage generator (11) and the inverted input terminal of the operational amplifier (14), a resistor (R2) (13) connected between the inverted output terminal of the operational amplifier (14) and the output terminal of the impedance converter (16), and an impedance element (capacitor) (15) connected between the output terminal of the operational amplifier (14) and the input terminal of the impedance converter (16), wherein a capacitor (17) to be measured is connected between the input terminal of the impedance converter (16) and a predetermined potential, and the capacitor measuring circuit (10) is provided in a position adjacent to the capacitor (17) to be measured.

Description

Description Capacitance detection circuit, capacitance detection device, and microphone device

The present invention relates to a circuit and a device for detecting a capacitance, and more particularly to a circuit, a device and a microphone device for detecting a minute capacitance with high accuracy. Background art

As a conventional example of the capacitance detection circuit, there is a circuit described in Japanese Patent Application Laid-Open No. Hei 9-1280806. FIG. 1 is a circuit diagram showing this capacitance detection circuit. In this detection circuit, a capacitance sensor 92 formed by the electrodes 90 and 91 is connected to an inverting input terminal of an operational amplifier 95 via a signal line 93. The capacitor 96 is connected between the output terminal of the operational amplifier 95 and the inverting input terminal, and the AC voltage Vac is applied to the non-inverting input terminal. The signal line 93 is covered with a shield line 94 and is electrically shielded from disturbance noise. The shield wire 94 is connected to the non-inverting input terminal of the operational amplifier 95. The output voltage Vd is taken out from the output terminal of the operational amplifier 95 via the transformer 97.

In this detection circuit, the inverting input terminal and the non-inverting input terminal of the operational amplifier 95 are in the state of image short, and the signal line 93 connected to the inverting input terminal and the shield line connected to the non-inverting input terminal 9 4 have almost the same potential as each other. As a result, the signal line 93 is guarded by the shield line 94, that is, the stray capacitance between the two is canceled, and an output voltage Vd which is hardly influenced by the stray capacitance is obtained. That can be is there.

However, according to such a conventional technique, when the capacitance of the capacitance sensor 92 is large to some extent, an accurate output voltage V d which is not affected by the stray capacitance between the signal line 93 and the shield line 94 is certainly obtained. However, there is a problem that the error increases when detecting a very small capacitance of the order of several pF or fF (femto-Farad).

Also, depending on the frequency of the applied AC voltage V ac, the tracking error in the operational amplifier 95 may cause a difference between the voltage at the inverting input terminal and the voltage at the non-inverting input terminal in the marginal short state. There is also a problem that the phase and amplitude shifts are delicate in nature, and the detection error increases. On the other hand, in light and small voice communication devices such as mobile phones, a compact amplifier circuit that converts voice detected by a capacitive sensor such as a capacitor microphone into an electrical signal with high sensitivity and high fidelity is required. If it is possible to accurately detect the required small capacitance of the order of several pF or fF or less or its change, it will be possible to detect sound with extremely high sensitivity and faithfulness A high-performance microphone is realized, and the performance of voice pickup in voice communication devices such as mobile phones is dramatically improved.

Accordingly, the present invention has been made in view of such a situation, and is capable of accurately detecting a minute capacitance, and is lightweight. ■ The capacitance of a condenser microphone or the like used in a small-sized voice communication device. It is an object of the present invention to provide a capacitance detection circuit or the like suitable for detecting the capacitance of a sensor. Disclosure of the invention

In order to achieve the above object, a capacitance detection circuit according to the present invention includes a capacitance detection circuit that outputs a detection signal corresponding to the capacitance of a capacitor to be detected. An impedance converter having a high input impedance and a low output impedance, a capacitive first impedance element, an operational amplifier, and applying an AC voltage to the operational amplifier. An AC voltage generator; and a signal output terminal connected to an output of the operational amplifier. An input terminal of the impedance converter has one end of the capacitor to be detected and one end of the first impedance element. The first impedance element and the impedance converter are included in a negative feedback path of the operational amplifier, and the detection capacitor and the capacitance detection circuit are provided adjacent to each other. It is characterized by having.

Further, the capacitance detection circuit according to the present invention is a capacitance detection circuit that outputs a detection signal corresponding to the capacitance of a capacitor to be detected, wherein the input impedance is high and the output impedance is low. A converter, a capacitive first impedance element, an operational amplifier, an AC voltage generator for applying an AC voltage to the operational amplifier, and a signal output terminal connected to an output of the operational amplifier. One end of the capacitor to be detected and one end of the first impedance element are connected to an input terminal of the impedance converter, and the first impedance element and the impedance converter are connected to a negative feedback path of the operational amplifier. Wherein the detected capacitor, the first impedance element, and the impedance converter are provided close to each other. And features.

Specific examples include an AC voltage generator, an operational amplifier whose non-inverting input terminal is connected to a predetermined potential, an impedance converter, and a circuit between the inverting input terminal of the operational amplifier and the output terminal of the impedance converter. The capacitance detection circuit comprises a connected resistor and a capacitor (first impedance element) connected between the output terminal of the operational amplifier and the input terminal of the impedance converter. The input terminal of the impedance converter Connected between potentials, the capacitance detection circuit and the capacitor to be detected are adjacent, or short and close so that the stray capacitance of the signal line does not exceed 10 times the maximum value of the capacitance of the connected element To be provided. Here, the predetermined potential refers to any one of a certain reference potential, a predetermined DC potential, a ground potential, and a floating state, and an optimum one is selected according to the embodiment. In addition, a resistor as a second impedance connected between the AC voltage generator and the inverting input terminal of the operational amplifier may be provided.

With this configuration, a constant voltage is applied to the capacitor to be detected, and almost all the current flowing through the capacitor to be detected flows to the capacitor (first impedance element), and the signal output terminal Outputs a signal corresponding to the capacitance of the detected capacitor.

In order to reduce noise in the signal line connecting the capacitance detection circuit and the capacitor to be detected, and to reduce the occurrence of stray capacitance in the signal line, the capacitor to be detected and the capacitance detection circuit must be connected to each other. Should be provided as close as possible to each other. Or, the capacitor to be detected, the first impedance element, and the impedance

— Install it as close to the dance converter as possible.

Here, in the specification of the present application, “nearly j” means that the stray capacitance of the signal line is larger than the capacitance value of the capacitance of the capacitor to be detected or the capacitance value of the capacitive first impedance element. This means that the stray capacitance of the signal line does not exceed the one-digit value of the capacitance value of the connected element. It has been found that the capacitance detection circuit can prevent the detection sensitivity from significantly deteriorating, and this has been obtained empirically. The capacitance can be measured by measuring the capacitance without connecting the capacitor, the first impedance element, and the impedance converter to the signal line. The state Contact.

Here, in addition to the capacitance detecting circuit, an inverting amplifier circuit for inverting a signal at a signal output terminal, an adding circuit for adding an output signal of the impedance converter and an output signal of the inverting amplifier circuit, May be added. Further, a resistor may be connected in parallel with the capacitor (first impedance element). Further, as an application of the present invention, the detected capacitor is a capacitive sensor that detects a physical quantity according to a change in capacitance, and the capacitance detecting circuit is formed on a printed circuit board or a silicon substrate. However, it is preferable that the capacitive sensor and the substrate are fixed, or that they are integrally formed. As a more specific example, more preferably, a capacitor microphone is used as the capacitor to be detected, and the capacitance detection circuit is realized by an IC, and the capacitor microphone and the IC are used. May be integrated and housed in a single case (shielded box) as a microphone used for mobile phones and the like. At this time, the capacitor microphone and the IC are fixed at adjacent positions, and are connected by a conductive plate, a wiring pattern, a wire bonding, or the like. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a circuit diagram of a conventional capacitance detection circuit.

FIG. 2 is a circuit diagram of the capacitance detection circuit according to the first embodiment of the present invention.

FIGS. 3A to 3E show examples of impedance converters usable in the present invention.

FIG. 4 is a circuit diagram of a capacitance detection circuit according to the second embodiment of the present invention.

FIG. 5 is a diagram (a cross-sectional view of a microphone) showing an application example of the capacitance detection circuit of the present invention to an electronic device. FIG. 6 is a schematic external view of the microphone shown in FIG. 4, in which (a) is a plan view, (b) is a front view, and (c) is a bottom view.

FIG. 7 is a cross-sectional view of another example of the microphone.

FIG. 8 is a schematic external view of the microphone shown in FIG. 6, in which (a) is a plan view and (b) is a front view.

FIG. 9 is a circuit diagram of a capacitance detection circuit according to another embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment)

FIG. 2 is a circuit diagram of the capacitance detection circuit 10 according to the first embodiment of the present invention. In this figure, the capacitance detection circuit 10 has a capacitor 17 to be detected (here, a capacitor microphone, etc., which detects various physical quantities by using a change in the capacitance Cs). (Capacitive sensor) is connected.

The capacitance detection circuit 10 includes an AC voltage generator 11 for generating an AC voltage, a resistor (R1) 12, a resistor (R2) 13, a operational amplifier 14, and an impedance element (here, It consists of a capacitor (capacitor Cf) 15 and an impedance converter 16, and outputs a detection signal (voltage V out) corresponding to the capacitance of the capacitor 17 to be detected from the signal output terminal 20.

One end of the AC voltage generator 11 is connected to a predetermined potential (ground in this example), and a constant AC voltage (voltage Vin, angular frequency ω) is generated from the other end (output terminal). A resistor (R 1) 12 is connected between the output terminal of the AC voltage generator 11 and the inverted input terminal of the operational amplifier 14.

Operational amplifier 14 has extremely high input impedance and open loop gain. This is a high voltage amplifier, in which the non-inverting input terminal is connected to a predetermined potential (ground in this example), and the non-inverting input terminal and the inverting input terminal are in the state of imagery short. The capacitor 15, the impedance converter 16, and the resistor (R 2) 13 are connected between the negative feedback path of the operational amplifier 14, that is, between the output terminal and the inverting input terminal of the operational amplifier 14. They are connected in series in this order.

The impedance converter 16 is a voltage amplifier whose input impedance is extremely high, whose output impedance is extremely low, and whose voltage gain is A times. The input terminal 21 of the impedance converter 16 is connected to the negative terminal of the detected capacitor 17 via a conductor such as a signal line or a wiring pattern on a printed circuit board. The other end of the capacitor 17 to be detected is connected to a predetermined potential (ground in this example). The output terminal of the operational amplifier 14 is connected to the output signal of the capacitance detection circuit 10, that is, the signal output terminal 20 for outputting the detection signal corresponding to the capacitance of the capacitor 17 to be detected. It has been. In the present application, each of the variables A indicated by A times or the like represents a real number other than zero (0).

Note that the connection between the capacitor 17 to be detected and the capacitance detection circuit 10 is possible in order to prevent unnecessary stray capacitance from being added as a detection error or from being mixed with disturbance noise. It is preferable to connect with the shortest conductor (cable, copper foil wiring pattern, connection terminal, etc.). In addition, if possible, cover the detected capacitor 17 and the capacitance detecting circuit 10 entirely with a grounded shield member, or provide a shield to enhance the shielding against disturbance noise. It is preferable to store it in a box.

The operation of the capacitance detection circuit 10 configured as described above is as follows.

It consists of resistance (R 1) 12, resistance (R 2) 13 and operational amplifier 14 Focusing on the inverting amplifier circuit, both input terminals of the operational amplifier 14 are in an imaginary short state and are at the same potential (for example, 0 V), and their input impedance is extremely high. Therefore, the current flowing through the resistor (R 1) 1 2 is Vin / R 1, and all of the current flows through the resistor (R 2) 13.Therefore, if the output voltage of the impedance converter 16 is V2,

V inZ R l =-V2 / R2

Is established. By rearranging this, the output voltage V 2 of the impedance converter 16 becomes

V 2 =-(R 2 / R 1)-V in (Equation 1)

Becomes In addition, since the voltage gain of the impedance converter 16 is A, the relationship between the input voltage (the voltage at the input terminal 21) VI and the output voltage (the voltage at the output terminal 22) V2 indicates that The input voltage VI is

V 1 = (1 / A) ■ V 2 (Equation 2)

Holds. Further, if the current flowing through the capacitor 15 toward the capacitor 17 to be detected is i, the input impedance of the impedance converter 16 is extremely high. 7, the current i becomes jωCs · VI, and the voltage Vout of the detection signal output from the signal output terminal 20 is

V out = i ■ (1 / j ω C f) + V 1

= (1 + Cs / Cf) ■ V 1 (Equation 3)

Becomes

From the above equations 1 and 2, when V2 is eliminated,

V 1 =-(R2 R 1) ■ (Vin / A) (Equation 4)

Is obtained. By substituting this V 1 into Equation 3 above,

V out =-(1 + C s / C f)-(R 2 / R 1)-(V in / A) (Equation 5) Is obtained.

As can be seen from Equation 5, the voltage Vout of the detection signal output from the signal output terminal 20 of the capacitance detection circuit 10 depends on the capacitance Cs of the capacitor 17 to be detected. . Therefore, by performing various signal processing on the voltage Vout, the capacitance Cs can be specified. Further, as can be seen from the fact that this equation 5 does not include the angular frequency ω, the voltage V out of the detection signal is equal to the frequency of the AC signal Vin from the AC voltage generator 11 and the frequency of the detected signal. It does not depend on changes in the capacitor frequency. As a result, the capacitance of the capacitor 17 to be detected can be detected without depending on the frequency of the AC voltage applied to the capacitor 17 to be detected. No) A capacitance detection circuit is realized. Therefore, for a capacitor 17 such as a condenser microphone whose capacitance value changes at a certain frequency (voice band), the capacitance value can be calculated directly from its voltage value without frequency correction of the detected signal. Can be specified.

Also, in the capacitance detection circuit 10 of the present embodiment, the operational amplifier 14 that supplies current to the capacitor 15 and the capacitor 17 to be detected has a non-inverting input terminal having a predetermined value. Connected to the potential and fixed. Therefore, unlike the operational amplifier 95 in the conventional circuit shown in FIG. 1, the operational amplifier 14 supplies a stable current with little noise without depending on the frequency of the input AC signal. Since the voltage is supplied to the capacitor 15 and the capacitor 17 to be detected, the minute capacitance of the capacitor 17 to be detected can be detected.

According to the experiment on the present invention, in the capacitance detecting circuit of FIG. 2, for example, the original capacitance of C s (capacitor to be detected: microphone in this embodiment) is 20 pF. When the stray capacitance of the signal line is 200 pF Beyond, the detection sensitivity deteriorated considerably. Further, when the above Cs was confirmed at several different capacitance values, the same tendency was obtained.

The capacitance C f and the detected capacitor C s, which are the first impedance element, are both capacitance elements connected to the signal line in this circuit, and the calculation is the same as above for both elements. It is likely to have consequences.

From these experimental results and experience, the detected capacitor, the first impedance element, and the impedance converter should be connected so that the stray capacitance of the signal line does not exceed the value of the capacitance of the relevant Cs or Cf by one order of magnitude. It was found that good detection sensitivity could be obtained when the distances were close to each other.

FIG. 3 shows a specific circuit example of the impedance converter 16 in the capacitance detection circuit 10 shown in FIG. FIG. 3A shows a voltage follower using the operational amplifier 100. Inversion of operational amplifier 100 The input terminal and the output terminal are short-circuited. By setting the non-inverting input terminal of the operational amplifier 100 as the input of the impedance converter 16 and the output terminal of the operational amplifier 100 as the output of the impedance converter 16, the input impedance is reduced. The impedance converter 16 is extremely high and has a voltage gain A of 1.

FIG. 3B shows a non-inverting amplifier circuit using the operational amplifier 101. A resistor (R10) 110 is connected between the inverting input terminal of the operational amplifier 101 and the ground, and a feedback resistor (resistance (R11)) is connected between the inverting input terminal and the output terminal of the operational amplifier 101. 3 3) is connected. By setting the non-inverting input terminal of the operational amplifier 101 as the input of the impedance converter 16 and the output terminal of the operational amplifier 101 as the output of the impedance converter 16, the input impedance is extremely low. As a result, an impedance converter 16 having a voltage gain A of (R10 + R11) ZR10 can be obtained. FIG. 3 (c) shows a circuit in which a buffer having a CMOS structure is added to the input stage of the operational amplifier as shown in FIGS. 3 (a) and 3 (b). As shown in the figure, an N-type MOSFET 34 and a P-type MOSFET 35 are connected in series between the positive and negative power supplies via resistors 112 and 113, and the output of the buffer is connected to the operational amplifier 100. (Or 101) input. By using the input of this buffer as the input of the impedance converter 16 and the output terminal of the operational amplifier as the output of the impedance converter 16, an impedance converter 16 with extremely high input impedance can be obtained. Can be

FIG. 3 (d) shows a circuit such as the buffer in the input stage of FIG. 3 (c). As shown in the figure, an N-type MOS FET 34 and a P-type MOS FET 35 are connected in series between the positive and negative power supplies, and an output is made from a connection between the two MOS FETs.

Figure 3 (e) shows the operational amplifier with the non-inverting input of the operational amplifier 102 as the input to the impedance converter, and connecting one end of a resistor 114 to the inverting input terminal of the operational amplifier 102. The output of 102 and the inverting input are connected via a resistor 115. As shown in FIGS. 3 (d) and 3 (e), by adopting such a configuration, an impedance converter 16 having an extremely high input impedance can be obtained.

(Second embodiment)

Next, a capacitance detection circuit according to a second embodiment of the present invention will be described.

FIG. 4 is a circuit diagram of the capacitance detection circuit 30 according to the second embodiment. This capacitance detection circuit 30 has a large size <a core section 31 corresponding to the capacitance detection circuit 10 shown in FIG. 2, and a signal output terminal 20 of the core section 31. Inverting section 32 that inverts with signal voltage V01 as input, and signal voltage V03 at output terminal 23 of inverting section 32 and alternating current of core section 31 It comprises an adder 33 that adds the signal voltage V 02 at the output terminal 22 and outputs a detection signal of the voltage V04 to the output terminal 24.

The core unit 31 is the same circuit as the capacitance detection circuit 10 shown in FIG. Therefore, the voltage V01 of the signal output terminal 20 of the core unit 31 is given by the above equation 5,

V 01 =-(1 + Cs Cf) ■ (R2 R 1) · (Vin A) (Equation 6), and the voltage V 02 of the AC output terminal 22 of the core 31 is calculated by the above equation 1.

V 02 =-(R 2 / R 1)-(V in / A) (Equation 7)

Becomes

The inverting section 32 is an inverting amplifier circuit including a variable resistor (R4) 40, a resistor (R5) 41, a variable resistor (R6) 42, a capacitor 43, and an operational amplifier 44. The variable resistor (R4) 4 is set so that the gain is 1 and the phase of the signal V03 at its output terminal 23 is the same as the signal V02 at the AC output terminal 22 of the core unit 31. The resistance values of 0 and the variable resistor (R6) 42 are adjusted. Therefore, the following relationship holds true between the input voltage V 01 and the output voltage V 03 of the inverting section 32.

V 03 =-V 01 (Equation 8)

The adder 33 is an adder in which three resistors (R7) 45, resistors (R8) 46, and resistors (R9) 47 having the same resistance value are connected to the operational amplifier 48. That is, the following relationship is established between the voltages V 02 and V 03 of the two input signals and the output voltage V 04.

V 04 =-(V 02+ V 03) (Equation 9)

Substituting Equation 8 above into Equation 9 to eliminate V03, then substituting Equations 6 and 7 above,

V 04 = V 01- V 02

= 1 (Cs / Cf)-(R2 / R1)-(Vin / A) (Equation 10) Holds. In other words, it can be seen that the voltage V04 of the detection signal output from the output terminal 24 of the capacitance detection circuit 30 is proportional to the capacitance value Cs. Therefore, by performing various signal processing based on the voltage V04, an unknown capacitance value Cs or a change in capacitance can be easily specified.

As can be seen by comparing Expression 10 with Expression 5 indicating the voltage Vout of the detection signal in the first embodiment, the capacitance is obtained by the capacitance detection circuit 30 in the second embodiment. Unlike the first embodiment, the detection signal includes only a component proportional to the capacitance of the capacitor 17 to be detected, and includes an unnecessary offset (a voltage independent of the capacitor 17 to be detected). Not in. Therefore, the signal processing for specifying the capacitance of the capacitor 17 to be detected from the detection signal in the second embodiment is simple.

Note that, in the present embodiment, an example in which V03 = —V01 has been described, but the present invention is not limited to this. Depending on the type of the capacitance sensor, the output voltage V04 can be expressed as V03 = k · V01 (k is the gain of the inverting amplifier)

V 04 = {k-(C s / C f) + (k + 1)} ■ (R 2 / R 1) ■ V in

You may set so that it may become.

FIG. 5 is a diagram showing an application example of the capacitance detection circuit in the first and second embodiments to an electronic device. Here, a cross-sectional view of a microphone 50 used for a mobile phone or the like in which a capacitor microphone and a capacitance detection circuit are integrated is shown. The microphone 50 includes a lid 51 having a sound hole 52, a vibrating membrane 53 vibrating by sound, a ring 54 fixing the vibrating membrane 53, and a speaker 55. a, a fixed electrode 56 provided opposite the vibrating membrane 53 via the spacer 55a, an insulating plate 55b supporting the fixed electrode 56, and an insulating plate 55b. The IC chip 58 fixed to the back surface and having the capacitance detection circuit of the above embodiment formed thereon, the IC package 59 molding the IC chip 58, the IC chip 58, and the like. It is composed of external electrodes 61a, 61b, etc. connected by bonding or the like.

The vibrating membrane 53, which is one of the electrodes forming the capacitor, is connected to a predetermined potential (ground in this example), and the fixed electrode 56, which is the other electrode, is made of an aluminum plate or wire bonding. It is connected to the circuit of the IC chip 58 through a conductor such as a contact hole. The capacitance of the capacitor composed of the vibrating membrane 53 and the fixed electrode 56 or its change is detected by the capacitance detection circuit in the adjacent IC chip 58 via the insulating plate 55b, and is detected by the electric signal. It is converted and output from the external electrodes 6 1a. The lid 51 is made of a metal such as aluminum, and has a conductive film (not shown) formed on the upper surface of the insulating substrate 60 as well as the internal capacitors 53, 56 and the IC chip 58. It plays a role as a shield box that blocks intrusion of disturbance noise. In this example, the fixed electrode 56 and the circuit are connected to each other, and the vibrating membrane 53 is connected to a predetermined potential. However, the vibrating membrane 53 and the circuit are connected to each other, and the fixed electrode 56 is connected to the predetermined potential. May be connected. However, empirically, the former is preferred.

FIG. 6 is a schematic external view of the microphone 50 shown in FIG. Fig. 6 (a) is a plan view, Fig. 6 (b) is a front view, and Fig. 6 (c) is a bottom view. The size of the lid 51 shown in FIGS. 6 (a) and 6 (b) is, for example, approximately 5 mm long and 2 mm high. The four external electrodes 61 a to 61 d shown in FIG. 6C are, for example, two terminals for the power supply of the capacitance detection circuit and two terminals for the output signal.

In such an application, the capacitor to be detected (here, a capacitor microphone) and the capacitance detection circuit (here, an IC chip) are provided adjacent to each other, and the signal line is extremely short. Capacitor microphone Capacitor microphone or first impedance element in circuit, whichever is greater It is connected by a conductor whose length does not exceed 10 times the quantity value. These components are covered with a shield member such as a metal lid. Therefore, in such an application example, it is considered that adverse effects such as disturbance noise mixed in the signal line (conductor) connecting the capacitor to be detected and the capacitance detection circuit can be ignored.

In other words, in such a small microphone, the capacitor to be detected and the capacitance detection circuit are connected by an extremely short conductor. Providing a special circuit for applying a guard voltage to the gate rather increases the circuit scale and prevents the circuit from being compact. Therefore, the shortest path between the capacitor to be detected and the capacitance detection circuit should be connected by a non-shielded (unshielded) conductive plate, wiring pattern, wire bonding, lead wire, etc. Preferably. As examples of other microphones, Figs. 7 and 8 show the circuit mounted on a board. This is basically the same as the above embodiment except that the capacitance detection circuit is mounted on the substrate 62.

As described above, the capacitance detection circuit according to the present invention has been described based on the two embodiments and the application to the product. However, the present invention is limited to these embodiments and the application. is not.

For example, in the capacitance detection circuits 10 and 30, a capacitor 15 is connected between the operational amplifier 14 and the impedance converter 16 to detect the current flowing through the capacitor 17 to be detected. However, it is also possible to consider connecting an impedance element such as a resistor or an inductance. Further, as shown in FIG. 9, a resistor 18 may be added in parallel with the capacitor 15 in the capacitance detection circuits 10 and 30 in the above embodiment and connected. As a result, the connection point between the capacitor 15 and the capacitor 17 to be detected is connected to the output terminal of the first operational amplifier 14 via the resistor 18. As a result, the DC floating state is eliminated, and the potential is fixed.

In addition, the capacitive sensors connected as the detected capacitors 17 are not limited to capacitor microphones, but also include acceleration sensors, seismometers, pressure sensors, displacement sensors, displacement meters, proximity sensors, touch sensors, and ion sensors. Sensor, humidity sensor, raindrop sensor, snow sensor, lightning sensor, alignment sensor, poor contact sensor, shape sensor, end point detection sensor, vibration sensor, ultrasonic sensor, angular velocity sensor, liquid level sensor, gas sensor, infrared sensor, Radiation sensors, water level gauges, freeze sensors, moisture meters, vibrometers, charge sensors, well-known capacitive sensors such as printed circuit board inspection machines, etc., all of which detect various physical quantities using changes in capacitance Of transducers (devices). As is apparent from the above description, the capacitance detection circuit, the capacitance detection device, and the microphone device according to the present invention apply the AC voltage to the operational amplifier via the resistor, and apply the detected voltage to the signal line. The capacitance of the capacitor to be detected is detected by connecting a capacitor. That is, a capacitor is connected between the output terminal of the operational amplifier whose non-inverting input terminal is connected to the predetermined potential and the input terminal of the impedance converter, and the input terminal of the impedance converter is connected to the predetermined potential. The detected capacitor is connected between them.

As a result, all of the current flowing through the capacitor to be detected flows into the capacitor, and an accurate signal corresponding to the capacitance of the capacitor to be detected is output to the output terminal of the operational amplifier. It is possible to detect minute capacitances of F order or less.

Then, the non-inverting input terminal of the operational amplifier is connected to a predetermined potential, and one potential of the input terminal is fixed, so that the operational amplifier operates stably, the operational error is reduced, and the detection signal is reduced. Included noise is suppressed.

A capacitor is connected between the operational amplifier and the impedance converter. As a result, detection sensitivity is ensured that does not depend on the frequency of the AC voltage applied to the operational amplifier and does not depend on the frequency of the capacitance change of the capacitor to be detected. Furthermore, when a resistor is connected between the operational amplifier and the impedance converter, there is no problem of deterioration of the SZN ratio due to thermal noise from the resistor.

The capacitance detection circuit and the capacitor to be detected may be provided adjacent to each other, or a circuit element connected to the signal line may be provided close to the capacitance detection circuit, and a shielded cable may be connected between them. Also, there is no need for a special circuit to cancel the stray capacitance generated in the cable.

Here, an inverting amplifier circuit for inverting the signal at the signal output terminal and an adding circuit for adding the output signal of the impedance converter and the output signal of the inverting amplifier circuit are added to the capacitance detecting circuit. You may. As a result, unnecessary offset components included in the output signal of the capacitance detection circuit are removed, and a net signal corresponding to the capacitance of the detected capacitor can be greatly amplified.

The capacitor to be detected is a condenser microphone, and the capacitance detection circuit is realized by an IC. The condenser microphone and the IC are integrated into one microphone as a microphone used in mobile phones and the like. Since the capacitor microphone and the capacitance detection circuit are placed very close to each other by being housed in a housing (shield box), the capacitor to be detected and the capacitance detection circuit are connected to each other. A special circuit, etc. for applying a large-diameter shield cap lug guard voltage is not required.

Further, since the capacitance detection circuit according to the present invention detects the capacitance by applying a current to the capacitor to be detected, the capacitance of the capacitor to be detected is, for example, an electret capacitor microphone. There is no need to attach a polymer film or the like to the Can be applied to sensors.

As described above, according to the present invention, it is possible to accurately detect a small capacitance with a limited use environment and realize a capacitance detection circuit and the like suitable for miniaturization. The voice performance of lightweight and small voice communication devices such as mobile phones has been dramatically improved, and their practical value is extremely high. Industrial applicability

INDUSTRIAL APPLICABILITY The capacitance detection circuit according to the present invention can be used as a detection circuit of a capacitive sensor, particularly as a microphone device provided in a small and lightweight device such as a mobile phone.

Claims

The scope of the claims

1. A capacitance detection circuit that outputs a detection signal corresponding to the capacitance of a capacitor to be detected,

An impedance converter having a high input impedance and a low output impedance, a capacitive first impedance element, an operational amplifier, an AC voltage generator for applying an AC voltage to the operational amplifier, and an output of the operational amplifier. A signal output terminal connected to the

One end of the detected capacitor and one end of the first impedance element are connected to an input terminal of the impedance converter,

A negative feedback path of the operational amplifier includes the first impedance element and the impedance converter;

The capacitor to be detected and the capacitance detection circuit are provided adjacent to each other.

A capacitance detection circuit characterized by the above-mentioned.

2. A capacitance detection circuit that outputs a detection signal corresponding to the capacitance of a capacitor to be detected,

One end of the capacitor to be detected and one end of the first impedance element are connected to an input terminal of the impedance converter,

The first impedance element and the impedance converter are included in a negative feedback path of the operational amplifier;

The detected capacitor, the first impedance element, and the impedance Close to the dance converter

A capacitance detection circuit characterized by the above-mentioned.

3. The capacitance detection circuit further includes a resistance element connected in parallel with the first impedance element.

The capacitance detection circuit according to claim 1 or 2, wherein:

4. A second impedance provided between the AC voltage generator and the operational amplifier is further provided.

The capacitance detection circuit according to any one of claims 1 to 3, wherein:

5. The capacitance detection circuit further comprises:

An inverting amplifier circuit for inverting a signal at the signal output terminal;

An adder circuit for adding an output signal of the impedance converter and an output signal of the inverting amplifier circuit;

The capacitance detection circuit according to any one of claims 1 to 4, characterized in that:

6. One end of the capacitor to be detected and the input terminal of the impedance converter are connected by a non-shielded conductor.

The capacitance detection circuit according to any one of claims 1 to 5, wherein:

7. The capacitor to be detected and the capacitance detection circuit are housed in one shield box The capacitance detection circuit according to any one of claims 1 to 6, wherein:

8. A capacitive sensor as the detected capacitor for detecting a physical quantity according to a change in capacitance;

The capacitance detection circuit according to any one of claims 1 to 7, wherein the capacitance detection circuit is formed on a print substrate or a silicon substrate, and is fixed to the capacitive sensor.

A capacitance detection device comprising:

9. A microphone device comprising: a capacitor microphone as the capacitor to be detected; and the capacitance detection circuit according to any one of claims 1 to 7.